Conventional acetabular cup prostheses employ a two-part construction comprising a cup portion and a separate bearing insert. The cup is secured in the acetabulum by one or more screws, which pass through the wall of the cup and directly into the bone of the acetabulum. Once the cup is secured, the bearing insert is installed in the cup. To prevent shifting or loosening of the cup, the acetabulum is reamed prior to cup insertion in order to provide a correct fit and a deeper pocket for the cup. To further improve stability, the cup is selected with a diameter large enough to span the widest part of the defect. The cup may also be selected to have a diameter slightly larger than that of the reamed acetabulum, such that a press-fit connection is achieved. However, a larger cup typically requires more reaming than a smaller cup, which may result in removal of viable as well as diseased bone.
Acetabular cup prostheses are used to correct various types of acetabular defects, including defects involving compromised acetabular walls or deficient bone. One type of acetabular defect is congenital hip dysplasia. In congenital hip dysplasia, portions of the rim of the acetabulum may be minimal and the acetabulum is shallower than normal. It is difficult to stabilize a cup prosthesis in a shallow acetabulum because little bone is available for reaming or for threading of screws.
Other deficient acetabular conditions may present problems similar to those encountered with dysplasia. In a revision hip replacement, loosening of the primary prosthesis or removal of bone cement often causes defects in the acetabulum. Infection may cause bone loss that results in a deformed or deficient acetabulum. Following a fracture, an acetabulum may heal in a deformed shape. In each of these situations, it may be desirable to minimize reaming and maximize use of available drillable bone in order to secure the prosthesis.
Conventional prostheses do not provide for secure fixation in deficient hips, such as those encountered in dysplasia, where there is not sufficient bone to allow for proper reaming and threading of screws. An inherent problem of prior art cups is a limited ability to be provided with fixation other than directly through the cup and into the acetabulum. To overcome this problem, attempts have been made to provide cups having an extension portion for use in securing the cup to available bone.
The prior art includes two-part acetabular cup prostheses that include rim extensions for use in securing the cups in deficient acetabula, such as the type encountered in congenital hip dysplasia. See U.S. Pat. No. 5,702,477 (Capello et al.); U.S. Pat. No. 5,931,870 (Cuckler); U.S. Pat. No. 6,162,257 (Gustilo et al.). These devices include screw holes in both the cup portion and the extension portion, such that the cup can be secured directly to the acetabulum and the extension portion can be secured directly to the surrounding bone, thus enhancing prosthesis stability even in deficient bone conditions.
U.S. Pat. No. 4,801,300 (Kurze et al.) recognized limitations in the ability of two-part hip prostheses to treat dysplasia hips. (Column 1, lines 41-42). To overcome this deficiency, Kurze et al. proposed a single-part hip joint socket provided with a perforated flange ring for mooring by implant screws. The flange covers at least two-thirds of the circumference of the hip joint socket. The flange has at least four uniformly distributed bore holes for receiving screws for securing the device on bone. The holes are unthreaded. Kurze et al. is directed primarily toward surface texturing for improving biocompatibility and mechanical stability. Kurze et al. provides no discussion of how the socket would be secured by screws. A disadvantage of Kurze et al. is that it does not provide for angulation of screws into available drillable bone in some deficient bone conditions.
FIG. 1 shows a prior art acetabular cup prostheses that is designed particularly for treatment of a dysplasic acetabulum. As shown in FIGS. 1A and 1B, the prior art cup has a pair of tabs that extend from an outer surface of the cup. Each tab includes a threaded hole for receiving a threaded screw for using in securing the cup in an acetabulum. As shown in FIGS. 1A and 1B, the tabs extend in the same plane as the rim of the cup, and are thus perpendicular to the axis of the cup. As shown in FIGS. 1A and 1B, the axis of each screw hole is parallel to the axis of the cup. One advantage of the acetabular cup of FIG. 1 is that it has a symmetrical configuration, and therefore can be used in either a left or right hip. However, a disadvantage of the acetabular cup of FIG. 1 is that the screws are oriented in a manner that does not maximize use of available drillable bone in some deficient bone conditions, such as those encountered in dysplasia.
GB Patent Application 2,347,864 (Paling) discloses a removeable attachment member that can be used to transform a conventional acetabular cup into a dysplasia cup. A primary objective of Paling is to allow a surgeon to determine intra-operatively whether to use a conventional cup or to convert the cup into a dysplasia cup. To accomplish this objective, Paling discloses an annular portion that is removably mountable on a rim of an acetabular cup and which has one or more integral flanges, each flange having a hole for receiving a screw. In order to negate the resultant moment imparted to the cup through the screws, the annular portion is preferably provided with two diametrically opposed flanges. Additionally, the two flanges are preferably angled relative to the annular portion. The flange angle is at a declination of 20 degrees or substantially 20 degrees from the underside of the annular portion. According to Paling, angulation allows a higher clamping force to be imparted by the ring when acting on the acetabular cup. However, as shown in FIG. 4 of Paling, the axes of the holes remains parallel to the axis of the cup, and thus do not provide for angled threading into bone. Paling discloses mounting the cup portion in the acetabulum in the conventional manner, followed by attachment of the annular portion to the cup portion such that projections on the annular portion engage recesses in the rim of the cup portion. Screws are then passed through the holes and into bone, such that the annular portion secures the acetabular cup in place. One disadvantage of Paling is that the annular portion is of thin construction, and therefore subject to potential failure. Additionally, because the annular portion is not fixed to the cup portion, there are circumstances in which the annular portion may separate or dislodge from the cup. Further, even in the angulated version of Paling, the screws are not angled so as to maximize use of available drillable bone in some deficient hip conditions.
There is thus a need for a acetabular cup prosthesis having the following characteristics and advantages over the prior art.